GPS Technology Rahul Jain Presentation
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GPS Technology Rahul Jain Presentation
- 1. Introduction to the Global Positioning System RAHUL JAIN rahuljaincse.blogspot.com
- 2. A brief history of the Global Positioning System Segments of the GPS A primer on how the GPS works Problems with the GPS Advancements in the GPS rahuljaincse.blogspot.com
- 3. IDEAS OF POSITIONING. EFFORT OF US DEPT.’S DEFENCE LAUNCHING DNSS.(DEFENCE NAVIGATION SATTELITE SYSTEM) EARLY PHASE 17 SETALLITES. PENTAGONS 24 SETALLITES. TADAYS 29 SETALLITES(BLOCK II VERSION) FIRST GPS SETALLITE 1978 PHASE I BLOCK I LATER IT WAS INCRESED BY 9 MORE SETALLITES. rahuljaincse.blogspot.com
- 4. Control Segment Space Segment User Segment Monitor Stations Ground Antennas Master Station rahuljaincse.blogspot.com
- 5. Kwajalein Atoll US Space Command Hawaii Ascension Is. Diego Garcia Cape Canaveral Ground AntennaMaster Control Station Monitor Station rahuljaincse.blogspot.com
- 7. Military.. Search and rescue. Disaster relief. Marine, aeronautical and terrestrial navigation. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). rahuljaincse.blogspot.com
- 8. Position and waypoint coordinates. The distance and direction between any two waypoints, or a position and a waypoint. Travel progress reports. Accurate time measurement. rahuljaincse.blogspot.com
- 9. it is received, multiplied by the speed of light, enables a GPS receiver to accurately calculate the distance between it and each satellite, provided that several factors are met. Those factors are: The receiver is somewhere on this sphere. rahuljaincse.blogspot.com
- 13. The Defense Department dithered the satellite time message, reducing position accuracy to some GPS users. S/A was designed to prevent America’s enemies from using GPS against us and our allies. In May 2000 the Pentagon reduced S/A to zero meters error. S/A could be reactivated at any time by the Pentagon. rahuljaincse.blogspot.com
- 14. Standard Positioning Service (SPS ): Civilian Users Source Amount of Error Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters Selective Availability (see notes) User error: Up to a kilometer or more Errors are cumulative and increased by PDOP. rahuljaincse.blogspot.com
- 15. Earth’s Atmosphere Solid Structures Metal Electro-magnetic Fields rahuljaincse.blogspot.com
- 16. User error = +- 1 km System and other flaws = < 9 meters rahuljaincse.blogspot.com
- 17. Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration. Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver. There are five distinct kinds of DOP. Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receiver’s position. It is usually up to the GPS receiver to pick satellites which provide the best position triangulation. More advanced GPS receivers can filter out poor DOP values. rahuljaincse.blogspot.com
- 22. DGPS Site x+30, y+60 x+5, y-3 True coordinates = x+0, y+0 Correction = x-5, y+3 DGPS correction = x+(30-5) and y+(60+3) True coordinates = x+25, y+63 x-5, y+3 DGPS ReceiverReceiver rahuljaincse.blogspot.com
- 23. USCG NDGPS Ground Stations National Differential Global Positioning System Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here. rahuljaincse.blogspot.com
- 24. USCG NDGPS Ground Stations National Differential Global Positioning System Yellow areas show overlap between NDGPS stations. Green areas are little to no coverage. Topography may also limit some areas of coverage depicted here. rahuljaincse.blogspot.com
- 25. Wide Area Augmentation System Geostationary WAAS satellites GPS Constellation WAAS Control Station (West Coast) Local Area System (LAAS) WAAS Control Station (East Coast) rahuljaincse.blogspot.com
- 26. How good is WAAS? + - 3 meters +-15 meters With Selective Availability set to zero, and under ideal conditions, a GPS receiver without WAAS can achieve fifteen meter accuracy most of the time.* Under ideal conditions a WAAS equipped GPS receiver can achieve three meter accuracy 95% of the time.* * Precision depends on good satellite geometry, open sky view, and no user induced errors. rahuljaincse.blogspot.com
Hinweis der Redaktion
- Three Segments of the Global Positioning System The Global Positioning System is comprised of three segments: the Control Segment, Space Segment and User Segment.
- Control Segment The Master Control Station, or MCS (also known as the Consolidated Satellite Operations Center) is located at the US Air Force Space Command Center at Schriever Air Force Base (formerly Falcon AFB) in Colorado Springs, Colorado. The MCS responsible for satellite control and overall system operations. The Control segment is made up of a Master Control Station (MCS), four monitor stations, and three ground antennas (plus a reserve antenna at Cape Canaveral used primarily for pre-launch satellite testing) used to uplink data to the satellites. Monitor Stations continuously receive GPS satellite transmissions, and relay this information in real time to the Master Control Station in Colorado. The user segment also receives these same transmissions. Monitor stations (MS) are located at Schriever Air Force Base, Hawaii, Kwajalein Atoll, and Diego Garcia, and Ascension islands. These stations are unmanned remote sensors that passively collect raw satellite signal data and re-transmit it in real time to the MCS for evaluation. Monitor stations basically function as very precise radio receivers, tracking each satellite as it comes into sky view. Ground antennas are remotely controlled by the MCS. They are also located at Ascension, Diego Garcia, Kwajalein Atoll, as well as Cape Canaveral, Florida. Ground antennas transmit data and commands from the Master Control Station to GPS satellites. The MCS uplinks data to GPS satellites, which includes: -Clock-correction factors for each satellite; necessary to insure that all satellites are operating at the same precise time (known as “GPS Time”). -Atmospheric data (to help correct most of the distortion caused by the GPS satellite signals passing through the ionosphere layer of the atmosphere). -Almanac, which is a log of all GPS satellite positions and health, and allows a GPS receiver to identify which satellites are in its hemisphere, and at what times. An almanac is like a schedule telling a GPS receiver when and where satellites will be overhead. Transmitted continuously by all satellites, the almanac allows GPS receivers to choose the best satellite signals to use to determine position. The almanac is automatically downloaded from satellites whenever a receiver is collecting a GPS signal. An almanac can also be downloaded from a computer, a base station or other archived almanac. -Ephemeris data is unique to each satellite, and provides highly accurate satellite position (orbit) information for that GPS satellite alone. It does not include information about the GPS constellation as a whole. Ephemeris information is also transmitted as a part of each satellite’s time signal. By using the information from the GPS satellite constellation almanac in conjunction with the ephemeris data from each satellite, the position of a GPS satellite can be very precisely determined for a given time.
- Space Segment The Space Segment is an earth-orbiting constellation of 24 active and five spare GPS satellites circling the earth in six orbital planes. Each satellite is oriented at an angle of 55 degrees to the equator. The nominal circular orbit is 20,200-kilometer (10,900 nautical miles) altitude. Each satellite completes one earth orbit every twelve hours (two orbits every 24 hours). That&apos;s an orbital speed of about 1.8 miles per second, and each satellite travels from horizon to horizon in about 2 hours. Each satellite has a design life of approximately 10 years, weighs about 2,000 pounds, and is approximately 17 feet across with its solar panels extended. Older satellites (designated Block II/IIA) still functioning are equipped with 2 cesium, and 2 rubidium atomic clocks. Newer satellites (Block IIR) are equipped with rubidium atomic clocks. All satellites also contain 3 nickel-cadmium batteries for backup power when a satellite is in earth eclipse (out of view of the sun). Each satellite transmits as part of its signal to ground stations and GPS receivers the following information: -Coded ranging signals (radio transmission time signals that allow a GPS receiver to triangulate its position). -Ephemeris position information (a message transmitted every 30 seconds containing precise information on the location of the satellite in space). -Atmospheric data (information to help correct interference of the signal as it travels through the earth’s atmosphere). -Clock correction information defining the precise time of satellite signal transmission (in GPS Time), and a correction parameter to convert GPS Time to Universal Coordinated Time (UTC). -An almanac containing information on the GPS constellation, which includes location and health of all the satellites. Whenever a GPS receiver is receiving a satellite signal it is automatically downloading an almanac. This almanac is stored in the receiver’s memory for future use. The stored almanac allows a receiver to more quickly acquire GPS satellite signals because it already knows the general location, and other information, about the satellites in the constellation. However, if a GPS receiver is left turned off for several months, or is moved more than 300 miles while turned off, the stored almanac may not be of any use to the receiver when it is turned on. A new almanac will be need to be downloaded for the receiver to function properly.
- The Four Basic Functions of the GPS The primary functions of the GPS fall into four categories: 1) Position and waypoint coordinates: A GPS receiver can provide position or waypoint information for its current location or any remote location on earth, and display that information in a variety of coordinates. 2) The distance and direction between a receiver’s position and a stored waypoint, or between two remote waypoints. 3) Velocity reports: Real time distance to any waypoint; tracking to a waypoint; heading (direction of travel); current speed; estimated time of arrival to a waypoint, course over ground, etc. 4) Accurate time measurement: GPS has become the universal timepiece, allowing any two receiver clocks (as well as any two clocks or watches) to be precisely synchronized anywhere in the world. The Global Positioning System operates using “GPS Time,” which varies slightly from Universal Coordinated Time (UTC). A GPS receiver corrects GPS Time anomaly to match UTC time (also known as “Zulu Time” or “Greenwich Time”), which is then offset by local time zone entered into the receiver by the user.
- How a Receiver Determines Its Position Traveling at the speed of light, each satellite PRN signal takes a brief, but measurable amount of time to reach a GPS receiver. The difference between when the signal is sent and the time it is received, multiplied by the speed of light, enables a GPS receiver to accurately calculate the distance between it and each satellite, provided that several factors are met. Those factors are: Good satellite signal lock by the GPS receiver (already covered) A minimum of four satellite signals (discussed next) Good satellite geometry (discussed later) When a GPS receiver is turned on it immediately begins searching the sky for satellite signals. If the receiver already has a current almanac (such as one acquired on a previous outing), it speeds up the process of locating the first satellite signal. Eventually it locates and acquires its first signal. Reading this signal the receiver collects the Navigation Message. If the receiver does not have a current almanac, it must collect a new almanac, which will take about 12-13 minutes after the first satellite signal is acquired. The almanac is automatically updated during normal use. In the above graphic, the GPS receiver has calculated a rough location that places it somewhere on the three dimensional sphere, which is actually thousands of miles in diameter. All the receiver can really do at this point is collect system data and search for more satellite signals.
- How a Receiver Determines Its Position (cont.) In a perfect world, where both satellite and receiver clocks would be perfectly synchronized with each other, an accurate position could be determined from just two satellites. However, most receivers are incapable of calculating an accurate position using just two satellites. The dot in the example represents the approximate location of where the receiver thinks it is based on the information provided by just two satellites. Now the receiver knows that it is somewhere at the intersection of those two satellite signals. But that’s the only improvement in its position calculations. The satellite signal spheres should intersect at precisely the receiver’s location, but don’t because the clock in the GPS receiver isn’t yet synchronized with GPS Time. So the receiver estimates a “pseudo-range” to each satellite. This in turn means that the yellow dot position shown in the example is still very crude. The area covered by the yellow on the earth’s surface is many miles wide. The receiver must continue to acquire more satellite signals to refine its position.
- How a Receiver Determines Its Position (cont.) For most receivers three satellites can only provide a two-dimensional (2D) position. Without manually entering the receiver’s exact elevation (most GPS receivers don’t allow elevation to be entered manually), the rendered 2D position may be off by several kilometers on the ground. If the exact elevation of the GPS receiver is known, entering that elevation into a receiver with this capability replaces the need for a fourth satellite signal to allow a receiver to triangulate a precise position. The receiver essentially uses elevation in lieu of a fourth satellite, and makes the appropriate adjustments to trilaterate a reasonably good 3D position. But without manual elevation correction most GPS receivers must rely on a fourth satellite to provide the final clock correction information necessary to calculate a 3D position. Until a fourth satellite signal is acquired the receiver will not be able to determine x and y horizontal, and z vertical positioning (a true 3D position). This is because the fourth satellite signal is used by the receiver not to provide more position data, but, rather, the final time correction factor in its ranging calculations. As a rule, 2D positions should always be avoided whenever possible. Use 2D positioning only when a 3D position is not possible, but be aware of the horizontal error inherent in any 2D position. The inability of a GPS receiver to triangulate a 3D position may be due to a variety of factors, including user error, poor satellite geometry, and harsh landscape conditions (tall buildings, canyons, and dense tree cover among others). As will be shown later in the course, all GPS receivers provide some means for informing the user which mode they are operating in. It’s up to the user to be aware of the errors associated with 2D positioning.
- How a receiver determines its position (cont.) For a GPS receiver to achieve three-dimensional (3D) positioning it needs to acquire four or more satellite signals. A 3D position is comprised of X and Y (horizontal), Z (vertical) positions, and precise time (not varying more than a few hundred nanoseconds). The receiver’s processor uses the fourth satellite pseudo-range as a timing cross check to estimate the discrepancy in its own ranging measurements and calculate the amount of time offset needed to bring its own clock in line with GPS Time (recall the radio station and record player simultaneously playing the same song). Since any offset from GPS Time will affect all its measurements, the receiver uses a few simple algebraic calculations to come up with a single correction factor that it can add or subtract from all its timing measurements that will cause all the satellite spheres to intersect at a single point (x, y, and z). That time correction synchronizes the receiver&apos;s clock with GPS Time. Now the receiver essentially has atomic clock accuracy with the time correction factor needed to achieve precise 3D positioning. The pseudo-ranges calculated by the GPS receiver will correspond to the four pseudo-range spheres surrounding the satellites, causing the four spheres to intersect at precisely the receiver’s location (the dot in the diagram).
- Selective Availability (Anti-Spoofing) Selective Availability (S/A) was the intentional degradation (referred to as “dithering”) of the Standard Positioning Service (SPS) signals by a time varying bias. Selective Availability is controlled by the Department of Defense to limit accuracy for non U. S. military and approved users. The potential accuracy of the coarse acquisition (C/A) code at around 30 meters was reduced by Selective Availability up to 100 meters. In May, 2000, the Pentagon set Selective Availability to zero. The Pentagon did not turn S/A off, but rather merely reduced the amount of signal dithering to zero meters, effectively eliminating intentional position errors for Standard Positioning Service users.
- Sources of GPS Error Satellite clock errors: Caused by slight discrepancies in each satellite’s four atomic clocks. Errors are monitored and corrected by the Master Control Station. Orbit errors: Satellite orbit (referred to as “satellite ephemeris”) pertains to the altitude, position and speed of the satellite. Satellite orbits vary due to gravitational pull and solar pressure fluctuations. Orbit errors are also monitored and corrected by the Master Control Station. Ionospheric interference: The ionosphere is the layer of the atmosphere from 50 to 500 km altitude that consists primarily of ionized air. Ionospheric interference causes the GPS satellite radio signals to be refracted as they pass through the earth’s atmosphere – causing the signals to slow down or speed up. This results in inaccurate position measurements by GPS receivers on the ground. Even though the satellite signals contain correction information for ionospheric interference, it can only remove about half of the possible 70 nanoseconds of delay, leaving potentially up to a ten meter horizontal error on the ground. GPS receivers also attempt to “average” the amount of signal speed reduction caused by the atmosphere when they calculate a position fix. But this works only to a point. Fortunately, error caused by atmospheric conditions is usually less than 10 meters. This source of error has been further reduced with the aid of the Wide Area Augmentation System (WAAS), a space and ground based augmentation to the GPS (to be covered later). Tropospheric interference: The troposphere is the lower layer of the earth’s atmosphere (below 13 km) that experiences the changes in temperature, pressure, and humidity associated with weather changes. GPS errors are largely due to water vapor in this layer of the atmosphere. Tropospheric interference is fairly insignificant to GPS. Receiver noise is simply the electromagnetic field that the receiver’s internal electronics generate when its turned on. Electromagnetic fields tend to distort radio waves. This affects the travel time of the GPS signals before they can be processed by the receiver. Remote antennas can help to alleviate this noise. This error cannot be corrected by the GPS receiver. Multipath interference is caused by reflected radio signals from surfaces near the GPS receiver that can either interfere with or be mistaken for the true signal that follows an uninterrupted path from a satellite. An example of multipath is the ghosting image that appears on a TV equipped with rabbit ear antennas. Multipath is difficult to detect and sometimes impossible for the user to avoid, or for the receiver to correct. Common sources of multipath includes car bodies, buildings, power lines and water. When using GPS in a vehicle, placing an external antenna on the roof of the vehicle will eliminate most signal interference caused by the vehicle. Using a GPS receiver placed on a dashboard will always have some multipath interference due to the surrounding metal and dashboard electronics.
- Sources of Signal Interference (cont.) Selective Availability (see previous slide). Control Segment blunders due to computer glitches or human error can cause position errors from several meters to hundreds of kilometers. Checks and balances by the Air Force Space Command virtually eliminates any blunders in the Control and Space segments of the Global Positioning System. User mistakes account for most GPS errors on the ground. Incorrect datum and typographic errors when inputting coordinates into a GPS receiver can result in errors up to many kilometers. Unknowingly relying on a 2D position instead of a 3D position can also result in substantial errors on the ground. A GPS receiver has no way to identify and correct user mistakes. Even the human body can cause signal interference. Holding a GPS receiver close to the body can block some satellite signals and hinder accurate positioning. If a GPS receiver must be hand held without benefit of an external antenna, facing to the south can help to alleviate signal blockage caused by the body because the majority of GPS satellites are oriented more in the earth&apos;s southern hemisphere. Errors in GPS are cumulative, and are compounded by position dilution of precision (PDOP) (covered later). It is the user’s responsibility to insure the accuracy of the data being collected with the GPS.
- Receiver Errors are Cumulative! As shown in the previous diagram, the Global Positioning System can contribute system errors of up to about nine meters on the ground. However, user errors can dramatically increase that to a kilometer or more. Among the most common user errors include using a 2D position instead of a 3D position, mismatching datums between a GPS receiver and a user’s map, and high position dilution of precision (PDOP). In the case of two dimensional (2D) positioning, the error occurs because the GPS receiver isn’t acquiring enough satellite signals (usually four or more) to provide a more precise position, or satellite geometry is so poor that a more refined position cannot be calculated. The difference can be as much as several kilometers from a user’s true location. Position dilution of precision (PDOP) will be covered later. The importance of not mismatching datums will be covered in the mapping section of the course. In the above example, the dark stars represent the potential positions a GPS receiver can triangulate for a user depending on the amount error incorporated into that receiver’s triangulation calculations. The large circle is a gross example of poor user control over the GPS receiver, and the smaller circle is the uncontrollable circle of error possible through system errors only. The centered star represents the hypothetical true location of the GPS receiver.
- GPS Satellite Geometry Affects the Level of Precision on the Ground Satellite position geometry can affect the quality of GPS signals and accuracy of receiver trilateration. Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver. There are five different kinds of DOP: GDOP: Geometric Dilution of Precision (refers to GPS satellite orbit anomalies) VDOP: Vertical Dilution of Precision (imperfect height measurements on the ground) HDOP: Horizontal Dilution of Precision (imperfect horizontal measurements on the ground) TDOP: Time Dilution of Precision (refers to slight anomalies in time measurements) PDOP: Position Dilution of Precision (three-dimensional position errors) Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the overall quality of a receiver’s position, and the quality of the data collected by the receiver. PDOP is specifically the calculated likelihood of position error based on the present position of the satellites being tracked, including horizontal and vertical errors. In other words, its HDOP and VDOP combined.
- Ideal Satellite Geometry Satellite geometry refers to the positions of satellites relative to each other in space. Dilution of Precision (DOP) is an indicator of the quality of a GPS receiver’s triangulated position relative to the quality of the geometric positions of the satellites whose signals the receiver is using. GPS receivers get satellite position information from the ephemeris message sent as part of the data stream from each satellite. Dilution of precision uses numerical values to represent the quality of satellite geometry, from 1 to over 100. The lower the number, the better the accuracy of position fixes. Some high-end GPS receivers (such as Trimble data loggers) have a default PDOP setting of around 8, and the value can be changed to meet the needs of the user. Garmin receivers do not allow PDOP manipulation by the user, nor do they provide a PDOP value. Instead they use estimated position error (EPE) value in feet or meters, which provides an estimate of the amount of horizontal error caused by poor satellite geometry. The outer ring of the circle in the above diagram represents the earth’s horizon. The center of the cross hair represents the sky directly above the GPS receiver. The satellite configuration shown is considered optimal for providing the best 3D positioning because any horizontal error from one direction will be offset by the opposing satellites. The fourth satellite directly overhead improves vertical accuracy.
- Good Satellite Geometry (cont.) The best satellite configuration for optimum receiver positioning is one satellite directly above the receiver, and three or more satellites equally spaced around the horizon (as shown in the previous diagram). In this configuration satellite geometry is very good since distance measurements are from all directions. The area where all four calculated pseudo-ranges intersect will be much smaller, providing a more precise position (as shown above). The farther apart the satellites are from each other (better geometry resulting in low PDOP) the smaller the red diamond will be in this diagram. Low PDOP results in a much smaller area of position uncertainty (the diamond in the diagram). A GPS receiver will choose a minimum of four satellites spread out across the sky rather than four satellites bunched close together in one quadrant of the sky. But it should be noted that many receiver types (Garmin included) always try to provide a position, even a poor position, regardless of satellite geometry. The reasoning is that any GPS position information provided by the receiver is better than no position information. This also explains why GPS receivers will provide a 2D (two-dimensional) position even though that position may be off by a considerable distance on the ground. Most GPS receivers provide some means of determining satellite geometry, either graphically (as in Garmin receivers), or through satellite position information (as in Trimble receivers). Garmin receivers use a sky view display to show the locations of GPS satellites above the receiver. A user can visually check satellite geometry, as well as the strength of the satellite signals, using Garmin’s initial startup screen. Other brands of receivers may use a similar or different method of displaying satellite geometry and signal strength. When using Trimble brand receivers, the best way to verify good satellite geometry is through the displayed PDOP value.
- Poor Satellite Geometry (Note: To properly view the animation in this diagram, use Slide Show feature of PowerPoint.) The locations of satellites in relation to each other in space at any given time can affect the quality of a GPS receiver’s position fix. Spaced low on the horizon, with no satellite directly above the receiver, can result in high PDOP. Similarly, if all satellites acquired by a receiver are bunched closely together in one quadrant of the sky can also result in poor triangulation measurements (and a high PDOP). Topography on the ground also affects satellite geometry. A receiver inside a vehicle, near tall buildings, under dense canopy, or in mountainous terrain can be affected by blocked signals. GPS receivers require clear line of sight to every satellite being acquired. The above diagram is a PowerPoint animation. Each part of the animation corresponds to the following sets: Satellite set 1: This satellite configuration results in poor PDOP and HDOP, but good VDOP. This is an example of a poor satellite configuration for achieving a precise position. Satellite set 2: This satellite configuration represents poor PDOP and VDOP, but good HDOP. It’s important to remember that satellite geometry that is poor for one kind of DOP can actually reduce another kind of DOP. If you need the best horizontal measurements, but don’t care about vertical accuracy, then this example is an acceptable satellite configuration. Satellite set 3: This satellite configuration represents poor PDOP, VDOP, and HDOP. This is another example of a poor satellite configuration.
- Satellite Geometry (cont) Most of the time there are more satellites available than a GPS receiver needs to triangulate a precise position fix, so the receiver can pick satellites which will provide reasonably good geometry for the receiver to triangulate a precise position. However, if a receiver happens to acquire satellites that are bunched close together in the sky, the intersecting circles that define a position will cross at very shallow angles. This increases the potential area of error around the receiver. In this diagram, two satellites close together will emit radio signals which cross at a shallow angle, producing a large area of potential error on the ground (the horizontal diamond in the above diagram). Add several more satellites with the same tight pattern, and the resulting PDOP will be very high.
- Real Time Differential GPS (This is an example of real time DGPS, and not a representation of post data collection differential correction done in the office on a computer using software such as Trimble’s Pathfinder office.) Real-time DGPS employs a second, stationary GPS receiver at a precisely measured spot (usually established through traditional survey methods). This receiver can correct many errors found in the GPS signals, including atmospheric distortion, orbital anomalies, Selective Availability (when it existed), and other errors. A DGPS station is able to do this because its processor already knows its precise location, and can easily determine the amount of error provided by the GPS signals by comparing its known location with the erroneous position data provided by the GPS. DGPS corrects or reduces the effects of: - Orbital errors - Atmospheric distortion - Selective Availability - Satellite clock errors - Receiver clock errors DGPS cannot correct for GPS receiver noise in the user’s receiver, multipath interference, and user mistakes. In order for DGPS to work properly, both the user’s receiver and the DGPS station receiver must be accessing the same satellite signals at the same time. This requires that the user’s receiver not be more than 300 miles from the DGPS station (100 miles or less is considered optimum). In the example, the DGPS station receives GPS signals telling the station that its location is x+5, y-3. But the station already knows that its true location is x+0, and y+0. So it calculates a correction of x-5, y+3, and transmits this correction out to the field on its own frequency. The DGPS receiver in the field uses this correction factor to update the same GPS radio signals its receiving. Here the GPS receiver triangulates its position with GPS as x+30, and y+60. The DGPS receiver provides the correction factor to the GPS receiver’s processor, which calculates its correct position at x+25, y+63. The user can generally get accurate position fixes within a few meters or less using DGPS. Many high-end GPS receivers have built in DGPS capability, while some low-end receivers (including some Garmin models) can be configured for DGPS using add on hardware. There are a number of free and subscription services available to provide DGPS corrections. The U.S. and Canadian Coast Guards and U.S. Army Corps of Engineers transmit DGPS corrections through marine beacon stations. The Wide Area Augmentation System (WAAS) is being developed by the Federal Aviation Administration (FAA) as another kind of highly advanced DGPS. This technology is currently being adopted by many GPS receiver manufacturers (including Garmin) to create more accurate receivers (more on WAAS later) .
- USCG NDGPS Ground Stations The map shows areas of likely US Coast Guard National Differential Global Positioning System (NDGPS) coverage by ground stations currently up and running in the U.S. These stations offer real-time differential GPS signals for receivers capable of receiving them. New NDGPS stations are coming online on a regular basis. The map shows areas of little or no differential GPS coverage in the United States, with Rocky Mountain states being most devoid of NDGPS coverage.
- USCG NDGPS Ground Stations (cont.)
- Wide Area Augmentation System The precision and accuracy of the Global Positioning System still limits its use for aircraft landings and in-flight navigation. As described earlier, satellite position errors, clock drift, and the earth’s atmosphere all compound GPS position information (both vertically and horizontally). However, the Federal Aviation Administration (FAA) realized the value in enhancing the GPS to provide for better aircraft navigation. Currently under development (the system is operational, but has not yet been approved for commercial civil aviation), the Wide Area Augmentation System (WAAS) is an experimental system designed to enhance and improve satellite navigation over the continental United States, and portions of Mexico and Canada. Unlike the GPS, which is funded and maintained by the U.S. military, the WAAS is funded by the FAA and Department of Transportation. It is specifically meant for civilian and commercial applications within the United States. Think of WAAS as a highly advanced real-time differential GPS. But instead of using ground based transmitters to broadcast position correction information, WAAS uses its own geostationary satellites in fixed orbit over North America. There are 25 ground reference stations positioned across the United States (including three in southern Alaska, and one each in Puerto Rico and Hawaii) that monitor GPS satellite signals. These stations continuously receive and correct GPS satellite information against their own known precise positions. Each WAAS ground station (referred to as a Wide Area Reference Station, or WRS) then sends its corrected GPS data to one of two master control stations located on the Pacific and Atlantic coasts of the U.S. These master control stations create a correction message that weeds out atmospheric distortion, GPS satellite orbit and clock errors and time errors. This message is then broadcast to the two WAAS satellites. These in turn re-broadcast the correction information using the basic GPS signal structure: L1 (“Link One”) on frequency 1575.42MHz. This allows any WAAS capable GPS receiver to pick up the corrected WAAS signal. In addition to the space based satellite WAAS, the FAA is installing ground based Local Area Augmentation System (LAAS) at most major airports in the U.S. Working in conjunction with the space segment of WAAS, the LAAS will allow for even greater accuracy for WAAS equipped aircraft during takeoffs and landings. The LAAS uses reference receivers located near airport runways, giving much more accurate correction information to incoming and outbound aircraft. However, currently the WAAS has not been approved for aviation by the FAA due to refinements still needed in the system to improve vertical and horizontal accuracy during Category 1 takeoffs and landings (low visibility conditions). Unlike differential GPS which requires additional equipment to work, the WAAS is available to anyone equipped with a WAAS capable GPS receiver in much of the United States and portions of Mexico. However, the System has its limitations at this time, including poor coverage over portions of the northern United States, and very slow signal acquisition time. WAAS capable GPS receivers are now widely available.
- How Good is WAAS? The Wide Area Augmentation System (WAAS) dramatically improves existing GPS technology for positional accuracy (in the United States and portions of Canada and Mexico). Under ideal conditions, with Selective Availability set to zero, horizontal accuracy with GPS can be fifteen meters or less. Under the same conditions with good WAAS signal acquisition that horizontal accuracy can be reduced to as low as three meters or less on the ground. Bear in mind that many factors dictate the level of accuracy that can be achieved by any GPS receiver on the ground. Among these factors include errors in the GPS, multipath interference, atmospheric errors, closed canopy or other signal blockers, and human error. Combined, these errors can degrade positional accuracy to 100 meters or more. For WAAS, two downsides are its reduced capability under heavy canopy (trees, canyons, etc.), and its limitation to mostly the contiguous U.S. In fact, some studies have shown that WAAS signals are degraded the further north from the 35 parallel one goes, reducing WAAS reliability in northern latitudes. - END PRE-WORK -